The present invention relates to an ultrasonic microscope system for measuring the acoustic characteristics of a sample through using an ultrasonic beam.
In recent years, there has been developed a mechanically scanning ultrasonic microscope for observing and measuring the microscopic or macropscopic structural and acoustic characteristics of a material through utilization of a focused ultrasonic beam, as disclosed in, for example, U.S. Pat. No. 4,028,933 issued on June 14, 1977 and "Acoustic Microscopy with Mechanical Scanning--A Review", Proceeding of the IEEE, Vol. 67, No. 8, August 1979, pp. 1092-1113. This ultrasonic microscope is, in principle, such one that applies a conically focused ultrasonic beam to a sample, moves the focal point of the beam in the plane of the sample, or in a direction perpendicular thereto, detects, by an ultrasonic transducer, reflected or transmitted waves caused by different elastic properties at respective points in the sample and converts them into electric signals for a two-dimensional display on a CRT screen to obtain an ultrasonic microscopic image, or for recording into an X-Y recorder or the like. Typical transducers for producing the focused ultrasonic beam are of the lens type and of the type in which an ultrasonic transducer is disposed on a concave or convex spherical surface. The ultrasonic microscopes are divided into the transmission type and the reflection type according to the location of the ultrasonic transducer. The abovesaid measurement is called imaging measurement by the ultrasonic microscope.
On the other hand, the abovesaid ultrasonic microscope has been modified for development of an acoustic velocity measuring apparatus. This type of apparatus is designed to observe the transducer output while moving a sample (a solid material, for example), towards the ultrasonic transducer along the beam axis (the Z-axis) instead of scanning in the X-axis and the Y-axis direction in the ultrasonic microscope. The transducer output is recorded as a periodically changing curve in a recorder. This curve is called a V(Z) curve, or an acoustic characteristic curve. It is well-known in the art that the periodicity of the V(Z) curve depends on the properties of the material to be measured and results from the interference between a reflected wave along the Z-axis of the focused ultrasonic beam directed to the sample and a reradiated wave of a leaky elastic wave excited by a beam component substantially at the critical angle. Accordingly, by measuring the dip interval .DELTA.Z representing the periodicity of the V(Z) curve, it is possible to know the acoustic characteristics of the material. The relation between the dip interval .DELTA.Z and velocity is given by the following equation, approximately: EQU .DELTA.Z=V.sub.l /{2f(1-cos .theta.)} (1) EQU .theta..sub.s =sin.sup.-1 (V.sub.l /V.sub.s) (2)
where .theta..sub.s is the critical angle, V.sub.l is the velocity of a longitudinal wave in a liquid acoustic field medium, V.sub.s is the velocity of the leaky elastic wave and f is the ultrasonic frequency used. Therefore, according to this acoustic velocity measuring apparatus, by measuring the interval .DELTA.Z, the velocity of the leaky elastic wave V.sub.s can be obtained from the following equation: EQU V.sub.s =V.sub.l /{1-(1-V.sub.l /2f.DELTA.Z).sup.2 }.sup.1/2( 3)
An example of this is disclosed in Weglein, "A Model for Predicting Acoustic Material Signatures". Applied Physics Letters, Feb. 1, 1979. Vol. 34, No. 3, pp. 179-181, and this article experimentally clarifies that this method is useful for the quantitative measurement of the acoustic characteristics of solids. This measurement is called quantitative measurement of the acoustic measurement of a sample by the ultrasonic microscope.
For the velocity determination by this measurement, it is necessary that the dip intervals .DELTA.Z in the V(Z) curve appear regularly. In general, however, the situation often arises where the dip intervals .DELTA.Z and the waveform of the V(Z) curve are so irregular that the dip intervals .DELTA.Z cannot be obtained from the curve, making it difficult to measure the velocity V.sub.s of the leaky elastic wave from the V(Z) curve.
The V(Z) curve recorded by the ultrasonic microscope apparatus includes every elastic information of the sample, and the abovesaid velocity measurement is an extraction of a part of the information. The V(Z) curve also includes an important factor which greatly affects the shape of an interference amplitude, that is, the propagation attenuation of the leaky elastic wave which contributes to the interference. It is considered that the amplitude attenuation of a leaky elastic wave which propagates on the boundary between the liquid acoustic field medium and the sample in the ultrasonic microscope apparatus is mainly caused by such three effects as (i) the radiation of the acoustic wave energy into the liquid owing to the acoustic loading of the liquid on the sample, (ii) the acoustic absorption by the sample and (iii) the scattering of acoustic waves due to the surface roughness of the sample and by the structural factors in the sample, such as cracks, pores and grain boundaries in the sample in which the leaky wave energy is distributed. Accordingly, by measuring the propagation attenuation of one or more leaky elastic waves which contribute to the V(Z) curve, it is possible to detect the acoustic impedance, the surface state and the internal structure of the sample.
In order to determine the propagation attenuation of the leaky elastic waves, two methods have been proposed and employed so far for the V(Z) curve obtained with the use of a conically focused ultrasonic beam. One of the methods is to estimate the attenuation by comparing the depths of dips or the magnitude of the interference amplitude in the measured V(Z) curve with those of the V(Z) curve obtained by theoretical calculations. The other method is to directly measure the attenuation of the leaky elastic wave amplitude, eliminating the response of the ultrasonic transducer to the ultrasonic beam component near the center axis of the beam in the V(Z) curve by attaching a sound absorber to an acoustic lens centrally thereof, using specially designed electrodes of the transducer, or employing an acoustic field suitable for the measurement so as to remove the beam component along the center axis of the beam, as described in, for instance, Smith et al., "SAW Attenuation Measurement in the Acoustic Microscope", ibid., 1982, 18, pp. 955-956.
However, these measuring methods have such fatal defects as follows: That is, the former method consumes much time for the comparison of the measured values with the calculated ones and insufficient in the correspondence between them because of the approximation by theoretical calculations, and hence is unsatisfactory in measurement accuracy. The latter method is inconvenient in that the velocity of the elastic wave must be measured using other methods such as the ordinary V(Z) curve method because the wave velocity is needed to determine the wave attenuation.